The natural fiber filament of a natural fiber reinforced plastic differs from the plastic it is reinforcing by having a relatively small contraction rate. In general, the plastic is susceptible to a considerable degree of shrinkage upon being heated. For example, an unsaturated polyester can shrink as much as 5-7% when heated. As a result, a natural fiber reinforced plastic board often bulges when heated. There are various inexpensive natural fibers, such as coconut fibers, palm fibers, hemp fibers, etc., that can be used to reinforce plastic. However, these inexpensive natural fibers have not been used economically and effectively to reinforce a composite material.